The present invention relates to a measurement system for measuring a low frequency electric field, the system comprising a capacitive antenna and a measurement device coupled thereto, the capacitive antenna comprising two parallel conductive plates, whereby an electric potential between the plates is a measure of the electric field.
Such a measurement system can, for example, be used for measuring unwanted low frequency electric fields generated by electronic devices. Sources of such unwanted low frequency electric fields are for example AC power-supplies fed from the AC supply, which generate unwanted fields at a frequency of 50/60 Hz and their harmonics, deflection coils in screens of television sets, which generate unwanted fields at a frequency of 15 kHz and its harmonics, and switch mode power supplies generating unwanted fields in the frequency band from 20 kHz to 200 kHz. Such unwanted fields can interfere with low frequency radio fields. Furthermore low frequency fields could be dangerous to people. Therefore, EMC-regulations are in force formulating requirements regarding the fields which electronic equipment is allowed to generate. Nowadays the awareness of the possible danger caused by such unwanted fields is growing. Therefore EMC-regulations are tending to become more severe. In order to check if electronic equipment fulfills such severe EMC-demands, measuring systems are needed which measure the fields very accurately. Furthermore the measurements carried out need to be reproducible.
In a known measurement system according to the preamble, which is used to measure low frequency electric fields generated by television sets, the capacitive antenna consists of a first plate having a length and width of some tens of centimeters and a smaller second plate parallel to the first plate. The antenna is connected to the measuring apparatus by a coaxial cable. One of the plates of the antenna is connected to the outer conductor of a coaxial cable, while the other plate is connected to the inner conductor thereof. This means that the electric potential between the plates is measured over the impedance of the coaxial cable, which impedance usually has a rather low value. Since the impedance caused by the capacitance between the two plates is much larger than the impedance of the cable, the signal measured over the impedance of the cable is only a small part of the potential present between the plates. Because of the noise also present over this impedance, it is impossible to accurately measure weak low frequency electric fields with this measurement system. Furthermore the use of an electrically conducting cable between the antenna and the measuring device introduces disturbances in the electric field to be measured. Since these disturbances are highly dependent on the exact position of the electrically conducting cable, this also considerably leads to a poor reproducibility of the measurements done with this system. Finally, the large first plate of the antenna also influences the electric field to be measured.